I can’t picture it. What is the expected product for each? If you draw a reaction mechanism, is there a clear difference? Does the fixed relative position of the double bonds in the cyclopentadiene help?

When in doubt, when someone says “why ether?” a good default answer is “non-polar solvent”.

Would potential carbocation rearrangement influence the speed of the reaction? That may happen w/ the butadiene but obviously not with the cyclopenta~. I also remember something specific about 1,3s that I learned late in organic 2, where you end up with a different arrangement than you’d expect, but I can’t recall it well enough to say whether it’s applicable here.

Is the cyclopentadiene reaction with Br2/light, or just Br2? If it’s Br2/light, it’s a radical reaction and would be much faster just because radicals are more reactive.

I bet it’s about the stability of the leftover anion; HCl is a weaker acid than HBR, which means that Cl- is less stable than Br-, so it’s probably reacting faster because the Chlorine anion reacts more violently to get an electron back. Just guessing.

Based on this additional information, I’m suggesting when he gets home, he draw the mechanism and see which has the more stable carbocation. That’s the key to reaction rate in SN1, if I recall. Resonance stabilization may be involved here. I am too lazy and have forgotten too much ochem to draw this out for my own self.

I’ve got to admit, I’m not overly thrilled with the ether question. If it were “why is ether used and not [some specific highly reactive chemical],” then sure. But, say, octane wouldn’t react with those things, either. We use ether because it does other things than just not react.

I’ve also got to admit, this organic stuff was surprisingly fun to come back to

Ok, I got another answer from my instructor in regards to these questions. Here are his comments

“The carbonium ion forms faster in the case of the first reaction above. The bromonium ion, which forms as an intermediate, has octets of electrons which make it more stable than any carbonium ion (see the last paragraph in Sec 5.6 on p 128. The more stable the cation the faster it forms. The second reaction reacts to form conventional carbocations.”

Also,

“Solvents may or may not participate in the reaction. In many instances, although the solvent doesn’t participate, it’s chemical characteristics are essential to the success of the reaction (e.g., aprotic solvents vs. protic solvents; polar solvents vs. nonpolar solvents, etc). In the present instance, ether, an aprotic, non-polar solvent is very inert and is not involved in any way in the reaction except as a “carrying” medium for the reactants.”